I have been enjoying the moon rise for 4 or 5 evenings recently. Our house is situated with mountains to the east so it is very easy to "mark" the spot on the horizon where the moon comes up each evening. QUESTION: Why is there such a great variation in the place on the horizontal plane where the moon rises from one evening to the next? There has been as much as 25 degrees or more difference in the spot that the moon clears the horizon from one evening to the next.

The position of Moonrise and Moonset, like that of Sunrise and Sunset varies as the Earth goes around the Sun, but also with the phases of the Moon.

Let's start with the position of Sunrise/Sunset, which varies as the Earth orbits the Sun. Because the Earth is inclined at an angle of 23.5 degrees to the plane of its orbit, and because the direction of the inclination (with respect to the stars) does not change as the Earth moves around the Sun, sometimes the Earth is tilted towards the Sun and sometimes it is tilted away from it. This cause the Sun to take different paths across the sky across the year and gives us seasons. In the Northern hemisphere the pattern of the position of Sunrise/Sunset is as follows (in the Southern hemisphere exchange North for South and vice versa):

Season

Position of Sunrise/Sunset

Winter

Southeast/Southwest

Spring

East/West

Summer

Northeast/Northwest

Autumn

East/West

Only on the equinoxes (Sept/Mar 21st) does the Sunrise/set at due East/West. At the solstices (Dec/June 21st) the position is its furthest South/North of East/West. How far to the North or South that is depends on your lattitude. There are other posted answers on this here, and here.

Now lets get to the Moon. The time of day that the Moon rises or sets depends on its phase. This should be obvious when you remember that the phase of the Moon depends on the relative positions of the Sun, Moon and Earth. For example when the Moon is Full it is opposite the Earth from the Sun, so when the Sun sets, the Moon must rise and vice versa. Here is a table summarizing that:

Moon phase

Moonrise

Moonset

New

Sunrise

Sunset

1st quarter

Local noon

Local midnight

Full

Sunset

Sunrise

3rd quarter

Local midnight

Local noon

By local noon and local midnight I mean the points when the Sun crosses the meridian, and exactly 12 hours later. This can be different from the time on your watch because we define time zones which all use the local time at the centre of the zone.

So when the Moon is new, it rises and sets with the Sun, and the position of Moonrise/set varies just like that of Sunrise/set. When the Moon is full however the pattern is inverted. To be more explicit (again here this is for the Northern hemisphere, for the South exchange North for South):

Season

Postion of Moonrise/set

NEW

1st

FULL

3rd

Winter

Southeast/Southwest

East/West

Northeast/Northwest

East/West

Spring

East/West

Northeast/Northwest

East/West

Southeast/Southwest

Summer

Northeast/Northwest

East/West

Southeast/Southwest

East/West

Autumn

East/West

Southeast/Southwest

East/West

Northeast/Northwest

Like the Sunrise/set positions, the amount of variation depends on your lattitude.

About the Author

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.

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